(eq 1.)
In fully aerobic soils, oxygen serves as the electron acceptor,
resulting in the production of CO2 and
H2O. The redox potential is controlled by the rates of
reduction and oxidation processes in soil and is connected to soil
respiration, for example in rewetted agricultural soils, where redox is
a strongly correlated predictor of CO2 flux (Bartolucci
et al., 2021). Based on this, redox could be a promising indicator for
soil carbon cycle (labile carbon pools) and structure (oxygen
availability), two key components of soil health (Kibblewhite et al.,
2008).
Redox can also serve as an indicator for nutrient availability and pest
regulation (Husson, 2013). Redox can be thought of as a parallel to pH,
which measures proton availability, as redox (Eh) measures electron
availability. And as pH, redox can influence nutrient availability
considerably. Redox is managed to avoid toxic As and Cd buildup in rice
paddies (Evans et al., 2021) and to improve Mn supply (Husson, 2013;
Zhang and Furman, 2021). A key challenge for using redox as an indicator
is however, that is changes rapidly with soil water and oxygen
conditions (Zhang and Furman, 2021). Husson et al. proposed to use redox
potential as a soil test from dried soil samples. (Husson et al., 2016).
In the test, dried soil would be rewetted and the redox would be
measured for 2 min. In theory, the test is similar to the
CO2 burst test, but taking only 2 min instead of 24 h.
It therefore has great potential for a high-throughput indicator for
soil health, but it should be tested in different soils and under
different management to see how it correlates with established soil
tests and how it reacts to management.
To evaluate redox as a soil biological health indicator, we compared it
to existing measures of soil health (CO2burst, visual
evaluation of soil structure VESS and soil organic matter). We used 18
sites from an ongoing carbon sequestration experiment (Carbon Action,
2019-; (Mattila et al., 2022)), where each site had a carbon farming
trial plot and a control plot. We measured CO2burst and
redox from dried and rewetted samples and compared the results with each
other and other analyses of soil properties. The results were used to
classify soils and to evaluate the change in soil health from three
years of carbon farming. This allowed the evaluation of the redox
potential as an indicator of soil health.